Neuroendocrine and Midgut Endocrine Systems in the Adult Mosquito

The Alphavirus Sindbis Infects Enteroendocrine Cells in the Midgut of Aedes aegypti

Transit of the arthropod-borne-virus (arbovirus) Sindbis (SINV) throughout adult female mosquitoes initiates with its attachment to the gut lumen, entry and amplification in midgut cells, followed by dissemination into the hemolymph. Free-mated adult females, aged day 5-7, were proffered a viremic blood suspension via sausage casings containing SINV-TaV-Green Fluorescent Protein (GFP) at a final titer of 106 PFU/mL. Midguts (MGs) from fully engorged mosquitoes were resected on days 5 and 7 post-bloodmeal, and immunolabeled using FMRFamide antibody against enteroendocrine cells (ECs) with a TX-Red secondary antibody. Following immunolabeling, the organs were investigated via laser confocal microscopy to identify the distribution of GFP and TX-Red. Infection using this reporter virus was observed as multiple GFP expression foci along the posterior midgut (PMG) epithelium and ECs were observed as TX-Red labeled cells scattered along the entire length of the MG. Our results demonstrated that SINVGFP did infect ECs, as indicated by the overlapping GFP and TX-Red channels shown as yellow in merged images. We propose that ECs may be involved in the SINV infection pathway in the mosquito MG. Due to the unique role that ECs have in the exocytosis of secretory granules from the MG and the apical-basolateral position of ECs in the PMG monolayer, we speculate that these cells may assist as a mechanism for arboviruses to cross the gut barriers. These findings suggest that MG ECs are involved in arbovirus infection of the invertebrate host.

Neuropeptidomics of the mosquito Aedes aegypti

Neuropeptidomic data were collected on the mosquito Ae. aegypti, which is considered the most tractable mosquito species for physiological and endocrine studies. The data were solely obtained by direct mass spectrometric profiling, including tandem fragmentation, of selected tissues from single specimens, which yielded a largely complete accounting of the putative bioactive neuropeptides; truncated neuropeptides with low abundance were not counted as mature peptides. Differential processing within the CNS was detected for the CAPA-precursor, and differential post-translational processing (pyroglutamate formation) was detected for AST-C and CAPA-PVK-2. For the first time in insects, we succeeded in the direct mass spectrometric profiling of midgut tissue which yielded a comprehensive and immediate overview of the peptides involved in the endocrine system of the gut. Head peptides which were earlier identified as the most abundant RFamides of Ae. aegypti, were not detected in any part of the CNS or midgut. This study provides a framework for future investigations on mosquito endocrinology and neurobiology. Given the high sequence similarity of neuropeptide precursors identified in other medically important mosquitoes, conclusions regarding the peptidome of Ae. aegypti likely are applicable to these mosquitoes.

Midgut ultrastructure of the third instar of Dermatobia hominis (Diptera: Cuterebridae) based on transmission electron microscopy

The midgut ultrastucture of the third-instar of Dermatobia hominis (L., Jr.) was investigated using transmission electron microscopy (TEM). The tubular midgut bears a monolayer of epithelial cells with the plasma membrane showing multiple folding where it adjoins the basement membrane. Septate junctions bound the epithelial cells on each side. These cells have electrolucent cytoplasm containing mitochondria, vacuoles, rough and smooth endoplasmic reticula, lamellar bodies, and a prominent nucleus with dispersed chromatin. The peritrophic matrix is close to elongate microvilli, which are sometimes forked. Regenerative cells, in an undifferentiated state when closest to the basement membrane, are scattered throughout the epithelial cells. A thick basement membrane, surrounded by thick connective tissue including muscle, tracheal tubes, and extracellular matrix is linked to epithelial cells by hemidesmosome-like structures. Entero-endocrine, goblet or cuprophilic cells were not observed.

Ultrastructure of endocrine cells from the abdominal midgut epithelium of Lutzomyia longipalpis (Diptera: Psychodidae)

Transmission electron microscopy (TEM) was used to study two types of endocrine cells front the midgut of adult female Luttzomyia longipalpis (Lutz & Neiva). Endocrine cells rarely have been observed in Nematocera, even using TEM, and were present in small numbers dispersed among the monolayer of midgut digestive epithelial cells. Triangular shaped "closed" cells were observed along the basement membrane, bounded on each side by digestive cells; these cells closed distally before reaching the epithelial lumen. These endocrine cells appeared to deliver active granules that were secreted through a cellular membrane into the hemolymph. A second cell type occupied a similar position to the closed cells, but opened into the midgut lumen via microvilli, where the secretory products may be delivered. Each cell type possessed both electron-lucent and electron-dense vesicles with secretory granules which may indicate different stages in maturation and activity. These granular secretory products are probably peptidergic substances, with secretion mediated by diet via basal and baso-lateral receptors that were bound to membranes or microvilli.

Localization of the mosquito insulin receptor homolog (MIR) in reproducing yellow fever mosquitoes (Aedes aegypti)

The female mosquito takes a blood meal to produce a batch of eggs. Initiation of egg maturation and growth of oocytes is governed by several endocrine factors. Peptide factors from the brain are involved in this process and some are also responsible for the induction of ecdysone secretion. The latter appears to be required to maintain a high rate of vitellogenin synthesis. By analogy with the known functions of insulin-like molecules (e.g. bombyxins) which in insects activate the secretion of ecdysteroids, we have postulated that there is an insulin receptor homolog responsible for activation of endysone secretion in the ovary. We have recently cloned the mosquito homolog (MIR) and are now investigating its spatial and temporal distribution. Here, we have localized the insulin receptor (MIR) both at the mRNA and protein level using in situ-hybridization and immunocytochemistry. The receptor is expressed before a blood meal mainly in the nurse cells of ovaries. After a meal, follicle and nurse cells contain mRNA coding for the receptor. The intensity of expression rises in the follicle cells until they degenerate during choriogenesis. Immunocytochemical localization confirms the in situ data: the protein is present before and after a meal. Both methods confirm our previous findings by Northern blot analysis, in which the ovary was found to be the main source of the receptor mRNA. The dynamics of receptor mRNA are related to the dynamics of ecdysone secretion and its action on physiological processes.

A temporal profile of the endocrine control of trypsin synthesis in the yellow fever mosquito, Aedes aegypti

Trypsin synthesis and secretion is induced after the female mosquito takes a blood meal. Its peak activity has been shown to be proportional to the amount and quality of food uptake. Further regulatory elements, hormones from the brain and the ovary, increase the synthethic rate of trypsin in the midgut by a factor of two. We investigated the temporal effect of removing the humoral factors by decapitation and ovariectomy. Trypsin synthesis was reduced to less than half its normal output when the operations were performed prior, or immediately after the blood meal. Postponing decapitation resulted in an increased activity. However, the dependence on hormones extended up to 14-16hrs after a meal, when maximal synthethic rates are assumed. Similarly, ovariectomy had a prolonged effect on trypsin synthesis. Finally, the lack of hormones reduced the synthetic capacity of the midgut even when small blood meals were given. We conclude that for continued efficient trypsin synthesis, humoral stimulation is necessary but is not part of the feedback mechanism that links the presence of food with the amount of trypsin secreted.

Plasmodium gallinaceum preferentially invades vesicular ATPase-expressing cells in Aedes aegypti midgut

Penetration of the mosquito midgut epithelium is obligatory for the further development of Plasmodium parasites. Therefore, blocking the parasite from invading the midgut wall disrupts the transmission of malaria. Despite such a pivotal role in malaria transmission, the cellular and molecular interactions that occur during the invasion are not understood. Here, we demonstrate that the ookinetes of Plasmodium gallinaceum, which is related closely to the human malaria parasite Plasmodium falciparum, selectively invade a cell type in the Aedes aegypti midgut. These cells, unlike the majority of the cells in the midgut, do not stain with a basophilic dye (toluidine blue) and are less osmiophilic. In addition, they contain minimal endoplasmic reticulum, lack secretory granules, and have few microvilli. Instead, these cells are highly vacuolated and express large amounts of vesicular ATPase. The enzyme is associated with the apical plasma membrane, cytoplasmic vesicles, and tubular extensions of the basal membrane of the invaded cells. The high cost of insecticide use in endemic areas and the emergence of drug resistant malaria parasites call for alternative approaches such as modifying the mosquito to block the transmission of malaria. One of the targets for such modification is the parasite receptor on midgut cells. A step toward the identification of this receptor is the realization that malaria parasites invade a special cell type in the mosquito midgut.

Early trypsin activity is part of the signal transduction system that activates transcription of the late trypsin gene in the midgut of the mosquito, Aedes aegypti

Trypsin activity during the first hours after feeding is essential to induce late trypsin gene expression. These results are consistent with the idea that free amino acids or other products released during digestion might be the initial signal for transcriptional activation of late trypsin. Besides early trypsin, some other factor(s) have to be translated for induction of late trypsin. This is the first case in which the proteolytic activity of a digestive enzyme is part of the signal transduction system which regulates expression of a second gene. The presence of two trypsins allows the mosquito to assess the quality of the meal and adjust the levels of late trypsin for a particular meal with remarkable flexibility.

Dietary control of late trypsin gene transcription in Aedes aegypti

In Aedes aegypti the levels of midgut trypsin activity after feeding are directly proportional to the protein concentration in the meal. The mechanisms of this up-regulatory event were investigated by analyzing the expression of the late trypsin gene under different dietary conditions. Transcription of the gene was dependent on both the quality and quantity of protein in the meal. As measured by Northern blot analysis, the levels of late trypsin gene expression increased up to 100-fold 24 h after feeding on gamma-globulin, hemoglobin or albumin (100 mg/ml). In contrast, gelatin, histone, amino acids, saline or agarose were very poor inducers of transcription. The rates of late trypsin transcription induced during the first 24 h were directly proportional to the concentration of protein in the meal. These data further support the suggestion that the primary mechanism that regulates the synthesis of trypsin in the mosquito midgut is transcriptional regulation of the gene. This regulatory mechanism enables the midgut to maintain the appropriate balance between protease synthesis and the protein content of the meal.